In many types of hearing aid, especially those of the in-the-ear type, sound from the receiver intended for the user's sound-sensitive organ may reach the microphone along an acoustical transmission path, the length and attenuation of which is so low, that positive acoustical feedback or "howling" may occur. This is especially the case with hearing aids of the in-the-ear type with a vent canal communicating the external auditory meatus with the atmosphere, as the sound from the receiver issuing into the meatus may be propagated along the vent canal and through the atmosphere to the microphone situated at a comparatively short distance from the vent canal.
Several attempts have been made or proposed to reduce the risk of positive acoustical feedback, but up to the present, none of these attempts have proved successful. Thus, attempts have been made by partly or completely occluding the vent canal, by introducing various filters, phase shifts and/or time delays or even negative feedback in the amplifying path, but all these attempts have led to discomfort to the user and/or reduced intelligibility of the speech processed by the hearing aid.